Solution Management During Decommissioning Of Heap Leach Operations

Heap leaching has become one of the prime technologies used for the recovery of precious metals from low-grade ores over the past ten years. The recovery process involves mining and stacking the ore on engineered leach pads, with or without crushing and/or agglomeration, followed by leaching with a low concentration sodium cyanide solution. The metal-bearing, pregnant solution is then processed by zinc precipitation or activated carbon absorption to extract the precious metals. The leaching process continues until all economic metal values are removed at which time the spent heap leach pads, the leached materials and excess process solutions require decommissioning. The management of these solutions may present a potential challenge for successful and environmentally sound decommissioning due to their composition, volume or continuity of generation from the spent ore even though active leaching has been terminated. The principal objectives of solution management in decommissioning spent heap leach pads are to meet all operative environmental regulations in a cost-effective manner and to leave the pads in a condition where they do not present a long-term liability either to the operator or to the environment. This involves the reduction in volume of the residual process solutions and/or treatment of these solutions to where they meet applicable water quality criteria. In the past, the environmental and economic aspects of decommissioning and solution management have been overlooked or under-estimated during the planning of mining operations. As a result, there is a potential for long-term liabilities and costs to be recognized only at the end of active leaching, a time at which operating revenues and cash-flow are minimal. Therefore, the decommissioning process requires addressing at the mine planning stage and agencies now require that a plan for solution management after the cessation of operations be submitted prior to initiation of mining. This paper discusses aspects of process solution management during decommissioning, both in terms of quantity and quality, and how potential issues may be recognized and planned for in the initial stages of heap leach design.